In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. To the demand for a resist material with a higher resolution and sensitivity, chemical amplification positive working resist materials which are catalyzed by acids generated upon light exposure are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Also, the change-over from i-line (365 nm) to shorter wavelength KrF excimer laser (248 nm) brought about a significant innovation. Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.30 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated.
For ArF excimer laser (193 nm), it is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 laser (157 nm) which is expected to enable further miniaturization to 0.10 μm or less, more difficulty arises in insuring transparency because it was found that acrylic resins which are used as the base resin for ArF are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. It was also found that poly(vinyl phenol) which is used as the base resin for KrF has a window for absorption in proximity to 160 nm, so the transmittance is somewhat improved, but far below the practical level.
It was reported in SPIE 2001, Vol. 4345, p. 273, that copolymers of tert-butyl α-trifluoromethylacrylate with 5-(2-hydroxy-2,2-bistrifluoromethyl)ethyl-2-norbornene and copolymers of tert-butyl α-trifluoromethylacrylate with 3-(hydroxybistrifluoromethyl)methylstyrene are appropriate resist polymers featuring high transparency and dry etching resistance. These polymers, however, have an absorbance of about 3. Only examples in which films of about 1,000 Å thick are patterned are reported therein. There is a need for further improvement in transmittance. It is generally believed that an absorbance of 2 or less is necessary to form a rectangular pattern at a film thickness of at least 2,000 Å. However, polymeric materials satisfying all the requirements of dry etching resistance, alkali solubility, substrate adherence and transparency have never been reported.
Furthermore, a highly transparent resin is described in SPIE 2002, Vol. 4690, p. 76. Owing to its absorbance of up to 1 and good substrate adherence, this resin is expected to find application at a film thickness of at least 2,000 Å. Since alcoholic groups are used as soluble groups, however, this resin has the drawback of a low dissolution rate in over-exposed areas where acid-eliminatable groups have been eliminated. For increasing the dissolution rate in over-exposed areas, it is contemplated effective to add a polymer which generates carboxylic acid or hexafluoroalcohol upon acid elimination.